One common method of forming single yarns has been the use of a spinning apparatus which drafts and twists prepared strands of fibers to form the desired yarn. One of the first yarn spinning apparatus was the mule spinning frame which was developed in 1782 and used for wool and cotton fibers. Many decades later, the ring spinning apparatus was developed to increase the spinning speed and quality of the spun yarn. Although good quality natural yarns may be produced by ring spinning, the rate of ring spinning remains relatively slow, e.g., less than about 15 meters/minute. In the last few decades, other various types of spinning apparatus which operate at higher speeds than ring spinning apparatus have been introduced. For example, rotor spinning, friction spinning and air-jet spinning methods are capable of spinning sliver into yarn at speeds greatly exceeding ring spinning speeds.
Prior to spinning sliver into yarn, the fibers are typically processed by carding and other various methods and then drawn to attenuate or increase the length per unit weight of the sliver. The sliver is generally drawn in a drafting zone comprising a series of drafting roll pairs with the speed of successive roll pairs increasing in the direction of sliver movement to draw the sliver down to the point where it approaches yarn width. Numerous parameters have traditionally been adjusted in the drafting zone to attempt to maximize the drafting and quality of the sliver including draft roll spacings, draft roll diameters, draft roll speeds (ratios), draft distribution, and fiber blending (e.g., drawframe and/or intimate blending).
One particular parameter, the draft roll spacing between adjacent roll pairs, is normally defined by the distance between the nip, i.e., the line or area of contact, between one pair of rolls and the nip of an adjacent pair of rolls.
The conventional wisdom for draft roll spacings, especially for higher speed spinning processes such as air jet spinning, has been to set the distance between adjacent nips at greater than the fiber length of the staple fibers in the sliver. See, e.g., U.S. Pat. No. 4,088,016 to Watson et al. and 25 U.S. Pat. No. 5,400,476 to White. This particular roll spacing has been widely accepted as the industry standard based on the rationale that smaller roll spacing results in increased breakage of fibers. Specifically, when the roll spacing is less than the fiber length, individual fibers may extend from one nip to an adjacent nip or bridge adjacent nips. Because adjacent pairs of rollers operate at different speeds, the bridged fibers may become pulled apart thus resulting in breakage of the fibers. This fiber breakage can result in low yarn quality and even yarn breakage in subsequent processing equipment such as spinning apparatus which may require the processing equipment to be shut down. Thus, draft roll spacings of greater than the fiber length have been the standard in the textile industry. The standard draft roll spacings produce yarns having good uniformity and mechanical properties. Nevertheless, there is always a need in the art to improve the uniformity and the mechanical properties of the yarn. Several attempts have been made to the drafting and spinning process to improve certain aspects of the spun yarn. For example, U.S. Pat. No. 5,481,863 to Ota describes decreasing the distance between the nip of the front roll pair of drafting rolls and the nip of the delivery rolls (located after spinning) to less than the longest fiber length to reduce ballooning in the air nozzles of the spinning apparatus. Additionally, U.S. Pat. No. 3,646,745 to Baldwin describes decreasing the distances between the nips of the front pair and the adjacent intermediate pair of drafting rolls to less than the effective staple length of the fibers in ring spinning processes to reduce the formation of "crackers" caused by overlength staple fibers. Nevertheless, no drafting takes place between the narrowly spaced rolls described in these patents and thus the problem of fiber breakage is not a danger in decreasing the roll spacings in these patents.
Parent application Ser. No. 08/844,463 ("the '463 application" disclosed that the uniformity and mechanical properties of spun yarn, particularly air-jet spun yarn, could be greatly enhanced by drafting sliver through a four-roll drafting zone in which the distance between the back roll pair and the adjacent intermediate roll pair, were both no more than the effective fiber length of the longest fiber type in the sliver.
Applicants have now additionally discovered, however, that yarn uniformity and mechanical properties can be similarly enhanced by maintaining the distance between the nip of intermediate roll pairs at no more than the effective fiber length of the longest fiber type in the sliver while maintaining a distance at the effective fiber length between the nip of the back roll pair and the nip of the adjacent intermediate roll pair.